This invention relates to a hydraulic servo vehicle brake with a master cylinder which contains at least one pedal-actuated master piston and at least one first brake circuit with wheel cylinders linked to the pressure chamber subjected to the pressure exerted by the master piston. A brake application valve is arranged between the brake pedal and the master piston and, on actuation by the brake pedal, applies pressure medium supplied by a hydraulic pump connected with a fluid reservoir in a controlled manner to the master piston and directly to a further brake circuit and furnishing a controlled pressure. A wheel slip-brake control unit which, in the event of incipient slip of one single or several vehicle wheels, automatically reduces the brake power at the relevant vehicle wheel or relevant vehicle wheels to a value just enabling the wheels to rotate. The control unit comprises a switching valve which is also subjected to the controlled pressure and is normally closed and opens at incipient slip of one or several vehicle wheels in order to apply the controlled pressure, through a non-return valve, directly to the pressure chamber on which the master piston acts. A hydraulic pedal-retaining arrangement which is subjected to the controlled pressure which penetrates through the opening switching valve, prevents any major change of the position the pedal has assumed at the moment of opening of the switching valve.
As soon as a wheel slip occurs in such a vehicle brake, the closing and opening valves, respectively provided at the wheel cylinders involved start to work cyclically in order to limit the brake pressure in the wheel cylinders to a value at which locking of the wheel is just avoided. Due to the cyclic functioning of the valves, a certain amount of pressure medium is consumed which requires balancing from the master cylinder in order to safeguard normal operation of the wheel cylinders.
It is known from German patent application No. 3,040,561 to convey the controlled pressure directly to the brake circuits past the sealing cup of one or both master pistons which is arranged as a valve in that the switching valve is commutated by the wheel slip-brake control unit in such a manner that the chambers before the sealing cup are linked to the controlled pressure instead of the fluid reservoir. In this known arrangement, the counterforce at the pedal is missing, so that the brake actuating pedal drops down and forward without providing any braking action and this will not only alarm the driver but also has the disadvantage that in the event of a failure of the auxiliary energy, a mechanical emergency operation of the brake would no longer be assured. For this reason, in the known vehicle brake, a positioning piston is provided which is also subjected to the controlled pressure through the switching valve and which moves in opposite direction to the master pistons when the switching valve is commutated. By means of stops the positioning piston prevents the primary or first master piston from sliding in the forward direction beyond a determined point as long as the auxiliary energy is available. In this manner, the brake pedal will not fully drop down when the switching valve responds, so that a sufficient pedal travel for an emergency stop will still be available in the event of a failure of the auxiliary energy.
It is, however, a disadvantage of the known vehicle brake that the positioning piston is required which has to be movable relative to the master piston and be sealed off by seals with respect both to the master piston and to the master cylinder. In addition, an increased overall space is required due to the arrangement and necessary play or motion of the positioning piston.
It is therefore an object of the present invention to create a vehicle brake of the type described by means of which drop down of the brake pedal when the switching valve responds is avoided without a positioning piston being required for that purpose.